You can fill an anonymously purchased Oyster with actual coins at select locations. I used to fill mine with otherwise useless small change periodically. In this way the Oyster is not tied to a credit card, although of course TFL does have CCTV so if they really wanted to (e.g. you murder somebody and leave the card at the scene) they could trawl through the CCTV footage matching the times the card was used.